The throw-off position allows, for example, the blanket of the blanket cylinder to be changed.
The throw-on position allows the printing unit to print the web of paper which passes between the pressing cylinder and the blanket cylinder.
In addition to the two positions, the blanket cylinder can generally occupy an intermediate position between the throw-off and throw-on positions thereof. In this position, which allows plates to be changed, the blanket cylinder is pressed against the plate cylinder and remains spaced apart from the pressing cylinder.
The throw-off position and the plate-changing position allow the web of paper to pass between the blanket cylinder and the pressing cylinder and thus to pass through the printing unit which is in the idle state. The web of paper can at the same time continue to be printed by other printing units.
This allows a printing operation to be carried out by some units within the same printing press whilst other units in the idle state are prepared for the following printing operation.
A press of this type is generally referred to as an “Auto Transfer” press.
FR-2-787 059 describes a dual printing unit in which the plate cylinder and blanket cylinder are each carried by levers which are articulated to the frame at eccentric points of the centre axes of the cylinders.
The movement of all the levers allows the above-mentioned positions to be reached.
Printing units are also known in which the ends of the cylinders are received in eccentric bearings in order to move them in order to reach the positions mentioned above.
Finally, U.S. Pat. No. 6,019,039 describes a printing unit of the above-mentioned type. The main connection rod can be moved with an eccentric movement relative to the axis of the plate cylinder. In this manner, a movement of the main connection rod relative to the frame always changes the distance between the axes of the blanket and plate cylinders. This kinematic system is therefore complex to implement, in particular if the plate cylinders and blanket cylinders had to be held one against the other in order to reach a plate-changing position.